There is a need for effective therapies to suppress pathogenic immune responses in autoimmune and alloimmune diseases. Our specific interest is in therapies that use antigen-presenting cells (APCs) as cellular tools for antigen-specific T cell immunomodulation. The present grant stems from the seminal observation made first by us, and later by others, that APCs can be converted into inhibitory cells, designated as "artificial veto cells (AVCs)," that can inhibit antigen-specific T cells. Our approach for engineering an AVC involves expressing (by protein transfer) a "coinhibitor" on an APC surface; this coinhibitor (such as a recombinant CD8 protein), operating coordinately with MHC.antigen, sends an inhibitory signal to the T cell. The present proposal consists of three specific aims which provide experimental paths towards determining the optimal surface display for an AVC and deciphering molecular determinants of coinhibitor function. These aims include: (1) characterizing the interplay between coinhibitors and costimulators using protein transfer and Abs as tools; (2) AVC engineering by protein transfer of lipidated derivatives of CD95L (Fas ligand) and other coinhibitor candidates; and (3) using isolated IgSF domains as probes for novel cell surface intermolecular interactions, with a focus on connections between CD8 and MHC proteins.